Here is my experience.
I ordered the 8 board cluster. Arrive some time last week.
After spending a great deal of time writing 8 disks, I tried booting
up each board one at a time, straight out of the box. Fearful of
USB hubs at this point, I used a serial connection alone (no USB,
no HDMI).
One of the boards failed ./test.sh.
It was one of the first
boards I tried. I continued with the rest and they all passed the
one run of ./test.sh. I returned to the first board and it passed.
And then tried again and it failed. Passed test intermittently, so
I wrote a script to run test.sh 100 times and record results.
So I installed heat sinks on 7 of the boards. ( One of the heatsinks
lost the backing in transit and was stuck to the plastic envelope
around the board in the box. Enough of the plastic stuck to the
adhesive when I pulled it free that I decided to not use it. Ordered
some thermal adhesive to attach later).
5 of the boards, including the one with no heat sink ran thru the
set of 100 test twice with no mismatch ERROR.
3 of the boards with heat sinks had double digit mismatch ERRORs.
I didn't have any computer fans around, so I found a 9 inch 60W
floor fan to create more than a little air flow.
With the air flow, the 3 troublesome boards all ran test.sh 100
times without ERRORs.
Will have to find a less obnoixous fan arrangement, but not sure
just yet how much air I need to move.
So you might get lucky and have success with not heat sink or fan,
but it appears to be the luck of the draw. I'm going to using multiple
power strips anyways, so I plan to power fans off AC.